Rare-earth (RE) doped, pulsed fiber lasers and amplifiers constitute efficient and compact optical sources that can emit a diffraction-limited Gaussian beam of highly controlled spectral quality. The output power generated by these sources is limited, however, by parasitic nonlinear optical effects, amplified spontaneous emission, and damage to optical components due to high optical power.
Nonlinear effects include stimulated Brillouin and Raman scattering (SBS and SRS), self- and cross-phase modulation (SPM and XPM), and four-wave mixing (FWM). The common origin of these effects is the high optical intensity in the fiber core and long path for the nonlinear interaction between the in-fiber optical beam and fiber material (e.g., silica). These effects hamper in particular the generation of high-peak-power pulses by causing unwanted spectral broadening, distortion of the pulse temporal profile, and sudden power instabilities that result in optical damages.
The build-up of amplified spontaneous emission (ASE) is due to the high optical gain available in the fiber core in the time interval between pulses. ASE constitutes an unwanted continuous-wave (CW) noise, which degrades the pulse/background contrast and, most importantly, limits the attainable pulse energy by using up gain.
Finally, optical damages can occur in the fiber because of material breakdown in the presence of high optical intensities. The fiber facets are especially vulnerable because exposed to potential contaminants and subject to defects that can initiate damage.
There is a need for fiber lasers and optical amplifiers configured to emit pulses of considerably higher energy and peak power than currently available. These sources must be designed so as to circumvent the limitations described above.